1. Field of the Invention
The present invention relates to a bass-reflex speaker, and more particularly relates to a bass-reflex speaker assembly that can be implemented in two-wheeled vehicles etc., vehicles that are used outdoors, and in other equipment.
2. Description of Related Art
Audio speaker systems where the phase of sound from a rear surface is inverted and the sound is outputted from a front surface are well known. This kind of speaker utilizes an acoustic effect where low-pitched sound resonates due to a hole formed at the front surface of a sealed speaker box and is referred to as a bass-reflex speaker. The resonant frequency is then determined by the size of the hole (port) and the length of a tube (resonant duct) attached to the port.
Ducts that project towards the outside of an enclosure (as in, for example, Utility Model Laid-open Publication No. Sho. 61-154092) are well known. Ducts that face towards the inside of an enclosure are suited to reproduction of low-pitched sounds, with resonant sound reflected at the rear surface of the enclosure being extracted from the duct. Ducts facing the outside function as a wall to prevent interference of high and low-pitched sound and are used to change frequency characteristics as a result of where such ducts are installed.
FIG. 8 is a view showing a duct that faces the inside of an enclosure and FIG. 9 is a cross-sectional view of a duct that projects outwards from the enclosure. In both drawings, a speaker 32 is attached to a panel 31 of the enclosure, and a duct 33 is positioned below the speaker 32. When a speaker system having this kind of configuration is mounted on a two-wheeled vehicle, etc., there are cases where water enters into the enclosure 30 from the duct 33 as a result of washing the vehicle or because of rain. Since water W temporarily entering the enclosure 30 accumulates at the bottom of the enclosure 30, it has, in reality, been difficult to adopt such a speaker system in a two-wheeled vehicle.
Attention should be paid to the following when audio speakers are mounted on a two-wheeled vehicle having a cowling. When the vehicle is in motion the inside of the cowling is at negative pressure and air pressure differences occur at the paper cones of the speakers, i.e., at both sides of a diaphragm. The paper cones therefore deviate towards the rider side and the sound quality deteriorates. Having a bias current corresponding to the air pressure difference flow through the voice coil has been considered in order to resolve this problem (for example, Japanese Patent Laid-open No. Sho. 63-211899). However, the method for providing a bias current makes the system more complex, and a simpler method is therefore preferred.